1. Field of the Invention
The invention relates generally to improving airflow in heat and mass transfer processes such as baking, freezing or drying and, more particularly, to improving impingement airflow by way of air fluctuation, increased airflow uniformity, and enhanced control of airflow. Embodiments of the invention are particularly applicable to use with ovens designed for baking pizza, especially restaurant ovens that can utilize the limited space available more efficiently and operate more quietly for a more compatible working environment.
2. Description of Related Art
Impingement processing that achieves heat and mass transfer modification of products is particularly attractive because impinging jets of air disrupt the layers of stagnant gas surrounding the surface of a product, thus increasing convection heat and mass transfer. The latter may also be increased by: vibration and pulsation of sound waves, rifling of air jets to be more cylindrical and less conical, rotating a jet about its axis, or swirling an air jet. See Huang, L. et al, "Heat Transfer and Flow Visualization Experiments of Swirling, Multi-Channel, and Conventional Impinging Jets," Int. J. Heat Mass Transfer, Vol. 41, No.3, pp.583-600 (1998), incorporated herein by reference, and placing a return air duct adjacent to air jet nozzles to make the jets more effective as described with respect to FIG. 1 of U.K. Patent No. 914,546, also incorporated herein by reference.
Many impingement ovens, for example, are hampered by a lack of uniformity of airflow due to design limitations. Nonuniform impingement airflow can cause irregularities within the oven, within baked products and between products being cooked at the same time. For example, the right-hand side of a pizza may be more baked than the left-hand side in some instances of nonuniform airflow, or identical raw pizzas may be baked to differing degrees in different zones of the same oven. Three important types of nonuniformity that adversely affect heat and mass transfer are: (1) differences in absolute air velocity at different nozzles due to irregular air flow into and out of the plenum chambers; (2) discharge of air from the nozzles in a non-perpendicular orientation to the surface of the heat and mass transfer modified product; and (3) tendency of the air jets to fan out into a widening cone more quickly than desired.
Past heat and mass transfer impingement devices have attempted to compensate for some of these deficiencies by passing air through long, thin, straight pipes which can impart a rifling effect to the discharged air. Air flowing through the pipes meets resistance to flow and experiences a pressure differential across the pipes. Expanding the pipe length aids in equilibrating the discharged air flowing from the pipes. Such processing equipment, however, tends to be very large and requires pipe lengths ranging from about 50-60 centimeters with pipe diameters of approximately two centimeters.
Impinging jets of gas in an oven can be expelled through nozzles in pipes through which gasflows under pressure, as described in U.K. Patent No. 985,443, incorporated herein by reference. Also, a high velocity jet of gas, so propelled, can be used to propel the flow of other gases by applying the Venturi effect, as exemplified by Newberry, Desmond, "Vacuum Cooling," in Proceedings of 72.sup.nd Annual Conference of the American Society of Bakery Engineers, Chicago, Ill., pp. 81-86, (March 1996), incorporated herein by reference. In both cases, airflow can be quieter and better controlled than with the use of plenum chambers into which air is propelled with the help of large fans and large air-circulation chambers. This is because the propulsion of gas under pressure can be regulated more conveniently by the adjustment of a valve opening than by the modulation of fan speed, without the inherent complexities of aerodynamic airflow within large chambers and the noise of the fan and said airflow within said chambers. Accordingly, an impingement system that uses compressed air in pipes with nozzles, or uses air propelled by the Venturi mechanism, promises to be quieter, more compact, and more easily controlled than circulating air with fans into plenum chambers.
One particular shortcoming of the prior art that is addressed by embodiments of the invention is the inability to employ impingement processing more effectively in applications where it is desirable to cook a baked product, such as pizza, traveling through the oven on a conveyor belt within the limiting space constraints of a restaurant kitchen. Even in those instances where a conveyor belt is used, only a single rack per oven would be feasible because of the blocking that would occur between vertically spaced baked products in typical ovens. Examples of ovens utilizing single-layer conveyor belts for cooking can be found in Ovadia, David Z., et al., "Impingement in Food Processing," Food Technology, Vol. 52, No.4, pp. 46-50 (April 1998), incorporated herein by reference.